Microporous films may be prepared by thermally induced phase separation (TIPS) processes. The noted processes typically involves extruding materials onto a casting surface. For example, the cast film may be a precursor to a microporous film where a molten solution of at least one crystallizable polymer (e.g., polyethylene) and a miscible diluent compound (such as mineral oil) is extruded from the die at a higher temperature (example: 400° F.) onto a lower temperature casting roll (example 150° F.). The resulting homogeneous solution phase separates when cooled after extrusion on the casting surface to form a two phase film of polymer and compound. The miscible compound (e.g., mineral oil) can be removed in subsequent processes such as solvent extraction etc. The resulting film can also be tentered and converted in subsequent processes to form a network of interconnected pores.